Heating rollers for web-like materials consist of a cylindrical roller body, at least one flange journal, at least one feedlline and/or discharge line for a heating medium, e.g. steam, and peripheral bore, lines or passages for passing the heating medium through the roller body. These rollers have a more or less solid roller body into which a larger number of axial bores have been made mostly near the roller surface, i.e., peripherally, a heating medium flowing through these peripheral bores and/or lines and transferring its thermal energy to the walls of the bores and/or lines and thus to the roller surface.
Another type of roller has a tubular roller body, in which the heating medium is passed through the hollow interior of the roller body, transferring its thermal energy to the interior of the roller body.
A significant difference between these two known types of heating rollers consists in that the heating roller type of the type first described above can certainly be used for liquid heating media such as water and thermal oil, but so far have not been used with an especially advantageous thermal transfer medium, namely steam.
This is due to the fact that steam condenses at least partly within the bores and/or lines, and transfers a great portion or all of its heat to the roller body, the roller surface and thus to the web-like material to be processed. The resultant condensate is pressed outwards due to the centrifugal forces mostly occurring in paper working machines, e.g. release super calenders, etc. into the bores and/or lines so that the condensate cannot flow off. Due to this, the bores are increasingly filled with condensate during operation until, finally, the entire bores are closed by water. Due to this, the flow rate of steam and, in parallel, the heating are practically reduced to almost zero. The known heating roller of the type first described above would have to be decelerated in this case to such an extent that the centrifugal forces are no longer sufficient for retaining the water in the bores and/or lines. In the extreme case, this type of roller would even have to be stopped in order to allow the condensate to flow off at least from the bores which are positioned at the top. It is obvious that situations occur at any rate both during the obstruction process and during the subsequent decelerating and emptying processes, which result in a heating roller of this type being heated non-uniformly so that irregular temperature profiles occur along the heating roller which lead to thermally caused deformation and distortions, etc.
For these reasons, steam has not been used as a heating medium for this type of roller. However, steam is an ideal heating medium, since it always condenses preferably at those locations within the roller and transfers its heating capacity to those locations which are the coldest locations. Thus, it is ensured by this property of the heating medium itself that there is always an approximate or uniform temperature profile across the surface which is essential for the treatment of the web-like material, or at the least, across the entire surface of the heating roller.
As opposed to this, the use of steam in the other type heating rollers does not present any problems, since the remaining condensate can be removed from the roller in known fashion either via a commercially available upright siphon or one rotating with the roller.
However, the roller of the type first described above has a decisive cost advantage as compared with the roller of the second type, namely, inasmuch as the heating rollers of the other type have a very large hollow space located in the roller body, and, for this reason, are subject to especially complicated acceptance conditions in many countries, e.g. in the USA, because they are considered to be pressure vessels. It must be borne in mind that the steam pressure is about 20 bar at a temperature of about 211.degree. C. and is about 40 bar at a temperature of about 249.degree. C. However, these complicated acceptance conditions do not apply to vessels having an internal diameter of less than 6 inches, i.e. less than about 152.4 mm., each of the individual axial bores and/or lines of the roller type first described above being considered as an individual vessel in the definition of the vessel.
Thus, consequently the advantage is that, with constant quality, functionality and operational safety, a roller of the type first described above can be produced and offered for sale at much lesser cost than a roller of the other type.
Moreover, the following must be taken into consideration: In order to comply with the line pressures which must be relatively high, e.g. in release supper calenders, namely up to about 450 or even 500 kN/M and, in individual cases, even higher, steel must be used as the material for the heating rollers, because for reasons of heat transfer to the paper, the roller wall must be as thin as possible. Moreover, the roller wall made of steel can be surface-hardened. However, the production cost for producing heating rollers of the other type is increased because of the special acceptance conditions.